diff --git a/src/tag.c b/src/tag.c
index 550d01a5..7b4030c8 100644
--- a/src/tag.c
+++ b/src/tag.c
@@ -15,6 +15,7 @@
 // Author: jdtang@google.com (Jonathan Tang)
 
 #include "gumbo.h"
+#include "util.h"
 
 #include <assert.h>
 #include <ctype.h>
@@ -60,14 +61,21 @@ void gumbo_tag_from_original_text(GumboStringPiece* text) {
   }
 }
 
+/*
+ * Override the `tolower` implementation in the perfect hash
+ * to use ours. We need a custom `tolower` that only does ASCII
+ * characters and is locale-independent to remain truthy to the
+ * standard
+ */
+#define tolower(c) gumbo_tolower(c)
 #include "tag_perf.h"
 
 static int
 case_memcmp(const char *s1, const char *s2, int n)
 {
 	while (n--) {
-		unsigned char c1 = tolower(*s1++);
-		unsigned char c2 = tolower(*s2++);
+		unsigned char c1 = gumbo_tolower(*s1++);
+		unsigned char c2 = gumbo_tolower(*s2++);
 		if (c1 != c2)
 			return (int)c1 - (int)c2;
 	}
diff --git a/src/util.h b/src/util.h
index c3edc27a..00d29907 100644
--- a/src/util.h
+++ b/src/util.h
@@ -57,6 +57,11 @@ static inline void gumbo_free(void *ptr)
   gumbo_user_free(ptr);
 }
 
+static inline int gumbo_tolower(int c)
+{
+  return c | ((c >= 'A' && c <= 'Z') << 5);
+}
+
 // Debug wrapper for printf, to make it easier to turn off debugging info when
 // required.
 void gumbo_debug(const char* format, ...);